An advanced Receiver Autonomous Integrity Monitor (ARAIM) is an important technology in the satellite navigation Augmentation System. As a typical onboard integrity monitoring technology, the ARAIM uses a redundancy measurement to quickly detect and eliminate the satellite faults and promptly alarm to the user.
The ARAIM allows a deviation of the descriptions for available σURA,i, σURE,i and bnom,i to be present in the measurement signal of the satellite; however, if the signal deviation goes beyond the described range, the satellite is believed to be faulted. The receiver judges which of the fault modes need to be monitored according to Psat,i and Pconst,j in the ISM information. Each fault mode corresponds to a subset in which an assumed faulted satellite signal is removed. The ARAIM ensures the navigation integrity by comparing the position solution of the subset against the position solution of the full visible satellite. If the solution separation amount of each subset solution and the position solution of the full visible satellite is within a predetermined threshold, the receiver obtains the following outputs by calculation: Protection Levels (PL), Effective Monitor Threshold (EMT), and Standard deviation of the accuracy (σacc). Wherein, the bound between the detection threshold sum and the subset solution covariance error has to be large enough to envelope the error of the full visible satellite position solution within the protection level. The EMT ensures the internal threshold to be rigorous enough. σacc provides a 99.99999% fault-free accuracy. The horizontal ARAIM(H-ARAIM) is one of three ARAIM working modes proposed in “the Second Milestone” issued by WG-C ARAIM Technical Team on February, 2015 (the other two modes are offline ARAIM and online ARAIM). The H-ARAIM mainly supports the ISM-based horizontal navigation and is extended to a multi-constellation based on the original RAIM technology. The multi-frequency allows the key integrity parameter to be altered through the ISM, while RAIM fixes these parameters within the receiver.
Currently, with the development of the global GNSS towards the multi-constellation, the multi-constellation H-ARAIM still presents the following problems: first, the classification for the fault subset is unreasonable; the existing ARAIM fault detection model is based on an idea of ergodic hypothesis; however, with the rapid increase of the number of the visible satellite, the calculation load also increases in folds, such that the availability prediction for the H-ARAIM cannot get the result quickly; second, the protection level estimation accuracy is low and the calculation speed is slow; in the calculation for the existing quality indexes such as the protection level and the like of the H-ARAIM, risks are averagely allocated to each fault subset; the obtained protection index level is not the optimal solution and is difficult to support the development requirements of the H-ARAIM and the related aerospace application.
For the first problem, scholars from Beijing University of Aeronautics and Astronautics propose a multi-constellation ARAIM fault detection mode based on an orbital plane. It is advantageous to quickly detect and eliminate faults by electing the constellation layer subset that fails the threshold test or has the largest test statistics to reduce the fault existence range, and thereafter electing the subset in the orbital plane corresponding to the constellation. With the premise of satisfying availability, the fault detection model using the orbit selection method for reducing the subset greatly simplifies the complexity of the ARAIM algorithm, reduces the calculation load, and is capable of ensuring the geometric integrity and is advantageous for quick detection and elimination of faults and real-time application.
For the second problem, with the modernization for GPS and GLONASS system and the development of BeiDou Navigation Satellite System (BDS) and Galileo, the number of the visible satellites certainly will increase, both navigation positioning accuracy and service reliability will be improved. However, more constellations and satellites also mean the increase of the possible fault modes of GNSS.
The reference MHSS algorithm is a commonly used user algorithm for ARAIM, which detects faults by comparing the solution of the full visible satellite and the subset solution assuming the removal of the faulted satellite. For the multi-constellation situation, a great number of subset solution in need of ARAIM receiver evaluation will be yielded, which greatly increases the calculation load.
For any phase of aviation flight, the safety risk limits that the aviation flight can bear in the corresponding standards are clearly established. Risks are from two aspects: one is an integrity risk, associating with a Missing Detection, (MD) probability; the other is a continuity risk, associating with a False Alarm (FA) probability. The more accurate the value of the protection level is, the user resistance against all of deviations may get more protection with a given risk value, and the higher the system availability is. Therefore, the protection level calculation relates to the allocation problems for the integrity and continuity risks, i.e., two extremely important and hark requirements that the civil aviation imposes on the satellite navigation system. The primary object for the optimization of the protection level calculation is to reasonably allocate the integrity risks and continuity risks of the satellite navigation system in each detection subset to obtain a more accurate protection level so as to realize the performance upgrade of H-ARAIM. The presently commonly used method for calculating the protection level are an average dichotomy in engineering and a theoretically target function method, however, in these two solving methods, the former one is too coarse to represent the reasonable allocation for the risks; the latter one is too complicated to use in the engineering application. On the other hand, the risk processing in the existing protection and calculation method is not suitable for the fault detection model using the orbit selection method. For the solution separation detection of the constellation layer subset solution, the continuity risks should be reasonably allocated at first, thereafter, the optimized allocation for the integrity risks should also be concerned when the protection level is solved at last.
Therefore, in order to address the abovementioned problems, there is a need for an H-ARAIM system of optimizing a horizontal protection level.